1. Field of the Invention
The present invention relates to an image forming device, and more particularly to an image forming device that uses a two-component developer containing a toner and a carrier.
2. Description of the Related Art
In an image forming device using an electrophotographic system such as a facsimile, a printer or a copying machine, an electrostatic latent image formed on the surface of a photosensitive member that serves as an electrostatic latent image carrying member is visualized by a developer, and the visualized image is transferred to a recording member such as a sheet and is thereafter fixed by heating and pressurizing. As the developer used here, a two-component developer containing a toner and a carrier is widely used in terms of high-quality and colorization.
In the development using the two-component developer, a developer carrying member incorporating a plurality of magnetic poles and carrying the developer on its surface is arranged apart and opposite the photosensitive member, in a region (development region) where the developer carrying member and the photosensitive member are opposite each other, a magnetic brush where the carrier is collected to rise is formed, a development bias voltage is applied between the developer carrying member and the photosensitive member to adhere the toner to the electrostatic latent image on the surface of the photosensitive member and thus the development is performed. Then, toner corresponding to the toner consumed in the development is sequentially supplied from a toner hopper to a development device, and thus the concentration of the toner within the development device is maintained within a predetermined range.
However, when an image, such as a solid image, having a high print rate is formed, a large amount of toner is consumed to greatly reduce the toner concentration around the developer carrying member, and thus it takes a long time to restore the toner concentration by the supply of the toner. In particular, in the case of long paper that is long in the direction in which a recording member is transported, it takes a long time to restore the toner concentration.
Hence, for example, patent document 1 (Japanese Unexamined Patent Application Publication No. 2005-91651) discloses that, when images are continuously formed on a plurality of sheets, even if a time for agitating a developer suitable for an interval between sheets of paper is adjusted according to the size of the sheet and the images are continuously formed on the sheets of long paper, the image quality is prevented from being reduced.
Moreover, patent document 2 (Japanese Unexamined Patent Application Publication No. 2008-268881) discloses a development device that includes: a developer supply portion (11a) provided in the developer supply position of a circulation transport room (3+4) where the developer is circularly transported; a developer consumption amount calculation unit for calculating the consumption amount of developer in a plurality of accumulative consumption amount calculation regions (A1 to A15) obtained by dividing the circulation transport room (3+4); a consumption amount transfer unit for transferring the consumption amount in each of the accumulative consumption amount calculation regions (A1 to A15) to the accumulative consumption amount calculation region (A1 to A15) on the downstream side in the developer transport direction according to the transport of the developer by a circulation transport member (7+8); a supply amount setting unit for setting a supply amount based on the consumption amount in the accumulative consumption amount calculation region (A1) corresponding to the developer supply position; and a developer supply control unit for controlling the supply of the developer based on the supply amount that has been set.
However, in the method disclosed in patent document 1, an image formation speed is likely to be reduced. Moreover, in the method disclosed in patent document 2, toner corresponding to the toner consumed, calculated from the number of pixels, is supplied later, and, when images having a high print rate are continuously formed on long paper, the toner concentration is significantly reduced at the back end of the sheet and on the downstream side in the transport direction of the developer in the direction of a developer roller shaft, with the result that the concentration is reduced.
The present invention is made in view of the conventional problems described above; an object of the present invention is to provide an image forming device that can maintain a toner concentration within a development device within a predetermined range without the toner being dispersed and an image formation speed being reduced even if an image having a high print rate is formed.